This invention relates to an electrical power generator and more specifically relates to a direct drive induction type generator which may be adaptable for use as a wind generator.
Wind turbine generators are coming into more frequent use as an alternative electrical power source. Wind farms use induction generators to convert the rotary movement of a wind turbine to electrical power. The fact that the wind velocity is random, unpredictable and subject to rapid changes complicates the manner in which the generator is connected to the AC power mains.
Induction machines are inherently capable of operating either as generators or motors, depending on the rotational velocity of the prime mover drive. For velocities greater than the machine's synchronous speed, the machine will operate in a generating mode and provide a power output to the mains. However, for velocities below the synchronous speed, the machine will operate in a motoring mode and draw power from the mains.
With the rising costs of fuel and ongoing environmental concerns, there has been considerable effort to develop new sources of electrical power. Included has been the development of systems primarily designed for providing power to a household, with any excess generated power being fed back to a power line of a public utility providing a primary source of power for that household. Frequently, the auxiliary or local power generating unit is in the form of a wind turbine, and there are times when little or insufficient power is available from it alone. Thus, as a matter of convenience, in order to reserve a continuous interconnection of power to on-site electrical devices to be powered, the windmill and public utility power lines are connected together.
Wind turbine generators have typically been of the direct current type, and thus in order to achieve compatibility with public power lines, which are of alternating current power, the output of such a generator must be converted to alternating current power. This is accomplished by switching means operating synchronously with the frequency, typically 60 cycles, of the power line. In addition to effecting frequency compatibility, there must be both voltage amplitude and phase compatibility between the generated output and the power line voltage. All in all, such a coupling system is necessarily complex and costly and reduces the overall system efficiency.
As an alternate to the direct current generator, induction motor/generator units are sometimes used with windmill generating systems. While the induction motor/generator has not seen great use as a generator in the past, it is perhaps the most widely used type of motor, and thus is widely available and at a reasonable cost.
The power input to an induction motor is given by the product of the applied voltage, the current, and the cosine of the phase angle between the voltage and current (E I Cosine a). In a heavily loaded motor, the current will tend to be in phase with the voltage. When unloaded, the current will typically lag the voltage 70 to 80 degrees. If an external force tends to drive the shaft higher than synchronous speed, the phase lag will continue to increase. When the force is sufficient to cause the phase lag to be 90 degrees, the power input to the motor is zero since cosine 90 degrees=0. At this point, the mechanical energy applied to the shaft is exactly equal to the magnetizing losses, and there is no net energy being generated. As the driving force continues to increase, the phase angle becomes greater than 90 degrees. The cosine of angles greater than 90 degrees is negative, indicating negative power flow. The motor is now generating power and returning energy to the buss. Further increase in driving force causes the phase lag to approach 180 degrees as the full generating capacity of the machine is reached.
Significantly, the induction generator requires no synchronization or voltage regulation circuitry to couple its output to a power line. It inherently functions as a generator when it is driven above its synchronization speed, a speed equal to the frequency of the power line divided by the number of pairs of poles that it contains, typically in the United States, the speed being 1,800 rpm in the case of a 4-pole device. It, like a direct current generator, is typically connected to a power line when its speed is sufficient for the production of power which, in the case of the induction motor/generator, is at sync speed. Beyond this speed, and in the range of approximately five percent of the sync speed, this type device provides increasing power output to a power line, this increase occurring as the phase lag of current with respect to voltage increases above 90 degrees, an angle which persists at the sync speed.
Prior art induction generators of this type typically employ a gearbox assembly to increase the speed of the rotor of the generator to match or exceed the synchronous speed to generate electricity. As mentioned above, for a 4-pole generator, the rotor must spin at 1800 rpm (7200/4=1800 rpm) to reach synchronous speed and begin to generate power. The use of a gearbox can be costly, add weight, require periodic maintenance, and reduce overall system efficiency.
As can be seen, there is a need for an improved power generator that harnesses the inherent abilities of an induction generator and also increases the overall power generation system efficiency so as to make power generation more cost effective.